Severe bronchial hyperresponsiveness to methacholine developed after intravenous therapy with OK-T3 and IL-2 in a patient with multiple myeloma, in whom no factors known to be associated with bronchial hyperresponsiveness were present. A substantial increase and activation of peripheral T-lymphocytes was observed after immunotherapy. Bronchial responsiveness and lymphocyte subsets both returned to normal baseline values 2 months after the patient was shifted to subcutaneous low dose administration of IL-2. The strict association between peripheral T-lymphocytes activation and the development of bronchial hyperresponsiveness suggests a causal relationship.